With the rapid growth of data-centric services, carrier providers are looking for implementing 100 Gbit/s Ethernet in the MAN or access network. The 100 Gbit/s or higher Ethernet architectures based on wavelength multiplexing for metro networks have been proposed, and an ETDM transmitter has been demonstrated recently for this system. However, the transmission of 100 Gbit/s signals per channel over wide-area network (˜100 km) will result in strong penalties from residual chromatic dispersion (CD) and polarization mode dispersion (PMD), even after practical optical impairment compensation. Moreover the total dispersion of the transmission system can be changed when the Ethernet signals are transmitted from one building to another building with different distance. For the new fiber with a length of 100 km and PMD coefficient of 0.1 ps/km1/2, the average PMD is about 1 ps and for the same transmission distances, older fibers with PMD coefficient up to 1 ps/km1/2 results average PMD of 10 ps. Moreover, some optical components such as optical couplers, optical multiplexers/demultiplexers may have large PMD. Therefore, it is important for 100 Gbit/s signals to have large CD and PMD tolerance.
Recently, orthogonal frequency division multiplexing (OFDM) modulation format has been proposed for optical communication system to improve the dispersion and PMD tolerance. This technology can simultaneously transmit signal in multiple sub-channels within a limited bandwidth. Therefore OFDM is high spectrum efficiency due to orthogonality between adjacent sub-channels, and the modulated signals can tolerate large fiber dispersion and PMD due to narrow spectral width of individual sub-channels.
The current method to generate OFDM signal for optical transmission is via signal processing in electrical domain. The spectrum of electrical OFDM over fiber transmission is shown in FIG. 1. Since the transmission signal for OFDM modulation is an analog signal, it requires high speed A/D and D/A converters. However, the bandwidth for the high-speed electronic devices required for electrical OFDM signal generation (such as A/D converter and D/A converter) is still limited currently. As a result, real time 100 Gbit/s OFDM transmission cannot be achieved physically by the commercial A/D or D/A.
Accordingly, there is need for a method for generating an optical channel of 100 Gbit/s or greater bandwidth with high spectral efficiency and dispersion tolerance.